Liquid crystal displays have been widely utilized for personal computer use, monitor use of portable devices and television use because of various advantages such as low voltage, low power consumption and possibility of miniaturization and thickness decreasing. In such liquid crystal displays, there have been proposed various modes according to the state of arrangement of liquid crystals in liquid crystal cells. However, a TN mode has hitherto prevailed in which a liquid crystal is twist-orientated through about 90 degrees from a lower substrate of a crystal cell toward an upper substrate thereof.
In general, the liquid crystal display comprises liquid crystal cells, optical compensating sheets and polarizers. The optical compensating sheet is used for eliminating image coloration and for enlarging viewing angles, and a stretched birefringent film or a film in which liquid crystals are applied onto a transparent film has been used. For example, Japanese Patent No. 2587398 discloses a technique for enlarging viewing angles by applying to a liquid crystal cell of the TN mode an optical compensating sheet in which discotic liquid crystals are applied onto a triacetyl cellulose film, orientated and fixed. However, in a liquid crystal display for TV use which is assumed to be viewed from various angles in a large-sized screen, demands towards viewing angle dependency are severe, and the demands have not been satisfied even by the technique as described above. Accordingly, liquid crystal displays of a mode different from the TN mode, such as an IPS (in-plane switching) mode, an OCB (optically compensatory bend) mode or a VA (vertically aligned) mode, have been studied. In particular, attention has been given to the VA mode as the liquid crystal display for TV use, because of its high contrast and relatively high production yield.
A cellulose acylate film is characterized by high optical isotropy (low retardation value), compared to other polymer films. Accordingly, the cellulose acylate film is generally used for use requiring optical isotropy, for example, for a polarizing plate.
On the other hand, optical anisotropy (high retardation value) is conversely required for the optical compensating sheet (retardation film) of the liquid crystal display. In particular, in the optical compensating sheet for VA mode, a retardation (Re) in a film plane of 30 to 200 nm and a retardation (Rth) in a thickness direction of 70 to 400 nm are required. Accordingly, as the optical compensating sheet, there has been generally used a synthetic polymer film having a high retardation value, such as a polycarbonate film or a polysulfone film.
As described above, in the technical fields of optical materials, it has been a general rule that when optical anisotropy (high retardation value) is required for a polymer film, a synthetic film is used, and that when optical isotropy (low retardation value) is required, a cellulose acetate film is used.
EP-A-911656 discloses a cellulose acetate film having a high retardation value which can also be used for use requiring optical isotropy, exploding the conventional general rule. In EP-A-911656, in order to realize a high retardation value in the cellulose acetate film, an aromatic compound having at least two aromatic rings, especially a compound having a 1,3,5-triazine ring, is added, and stretching treatment is performed. In general, it is known that cellulose triacetate is a polymer material difficult to be stretched, so that it is difficult to increase birefringence. However, it is made possible to increase birefringence by concurrently orientating the additive by the stretching treatment, thereby realizing a high retardation value. This film can also serve as a protective layer for a polarizing plate, so that it has the advantage that an inexpensive and thin-thickness liquid crystal display can be provided.
JP-A-2002-71957 discloses an optical film containing a cellulose ester which has acyl groups having 2 to 4 carbon atoms as substituent groups and meets the equation: 2.0≦A+B≦3.0 and the equation: A<2.4 at the same time, when the degree of substitution of acetyl groups is taken as A, and the degree of substitution of propionyl or butyryl groups is taken as B, wherein the refractive index (Nx) in a slow axis direction and the refractive index (Ny) in a fast axis direction at a wavelength of 590 nm meet the equation: 0.0005≦Nx−Ny≦0.0050. JP-A-2002-270442 discloses a polarizing plate used in a VA mode liquid crystal display, wherein the polarizing plate has a polarizer and an optically biaxial mixed fatty acid cellulose ester film, and the optically biaxial mixed fatty acid cellulose ester film is arranged between a liquid crystal cell and the polarizer.
The above-mentioned method is effective in that the inexpensive and thin liquid crystal display is obtained. However, a higher retardation value has recently been required, which makes it necessary to increase the amount of a retardation increasing agent added or to increase the stretch ratio. However, it has become difficult to realize a desired retardation value because of bleeding out of additives or breakage of the film in stretching. Further, the liquid crystal displays are increasingly frequently used under various environments, and the cellulose ester film used in the above-mentioned technique has the problem that the optical compensating function thereof varies under the environments. In particular, there has been the problem that when the cellulose ester film is bonded to the liquid crystal cell, it is influenced by changes in the environment, particularly changes in humidity, to cause changes in Re retardation value and Rth retardation value thereof, resulting in changes in optical compensating ability. It has been desired to solve this problem.